Several aspects of the interaction of the complement system with Actinomyces viscosus T14V and A. naeslundii WVU45, organisms implicated in the pathogenesis of periodontal disease, are under exploration. Complement fixation by monoclonal antibodies has been used to localize epitopes on the fimbriae of these bacteria. These studies have provided evidence that the subunit structure of the type 1 fimbriae (which mediate attchment of A. viscosus T14V to saliva treated hydroxyapatite) and the type 2 fimbriae (which are associated with lectin activity for saccharide receptors on bacteria and mammalian cells) differ. Thus, each of three monoclonal antibodies specific for the type 1 fimbriae in the absence of a second antibody activates the complement sequence. In contrast, only one of four monoclonal antibodies specific for the type 2 fimbriae (all of which consume complement when assayed in combination with a second antibody) activates this mediator system. Since complement activation by IgG antibodies is strictly dependent on the bridging of two antibody molecules by C1q within a given distance, the type 2 fimbriae are apparently more extended than the type 1 fimbriae, a finding which has been suggested by recent electron microscopic examination of the fibriae. The biological activity of one monoclonal antibody can clearly be influenced by the addition of a second antibody. Combination of monoconal antibiodies has revealed inhibition, enhancement, addition and synergy of complement activation. Investigation of complement and non-complement dependent destruction of the actinomyces by polymorphonuclear leukocytes has revealed multiple mechanisms. The lectin associated with the type 2 fimbriae can initiate phagocytosis by attaching to carbohydrate containing receptors exposed by sialidase treatment of PMNs. This interaction is abolished by the addition of lactose. Antibodies reactive with the cell wall or the two types of fimbriae are effective opsonins for the induction of Fc receptor mediated phagocytosis. In addition, C3b/C3bi deposited on the bacterial cell wall by antibody initiated activation of the complement cascade enhances antibody mediated phagocytosis.